Constitutive and regulated luciferase expression in vivo. (a) Plasmid pcLuc+ (15 μg) was injected intramuscularly into the right quadriceps of six naïve DBA/1 mice. The muscle of three mice was then electroporated (8 pulses, 200 V/cm, 20 ms duration, 2 Hz), and the other three mice were untreated. Three days later the experiment was terminated and muscle processed for measurement of luciferase. Levels of luciferase are the mean of three animals and are normalized for protein concentration of the muscle lysate. Vertical lines represent standard error, and a significant difference (P ≤ 0.01) between the luciferase level in the muscle of electroporated and non-electroporated mice is indicated (*). (b) Plasmid DNA (15 μg) was injected (intramuscularly) and electroporated in naïve DBA/1 mice. After 2 weeks GTRTL injected mice (n = 5 for all groups) were given 10% sucrose (white bar), or doxycycline drinks (black bars) prepared in 10% sucrose at 200 μg/ml and 2 mg/ml for the subsequent 2 weeks. Luciferase expression in muscle was determined in dissected muscle and was compared with that in mice receiving the control plasmids pGCMV, pGmCMV and pGTL. Significant differences between the pGTRTL plus doxycycline groups and the non-induced pGTRTL group (P ≤ 0.05) are indicated (*). (c) Downregulation of luciferase expression from pGTRTL was assessed in a group of mice that had received doxycycline 200 μg/ml for 2 weeks; they were switched to sucrose for 3 days, after which luciferase levels in muscle were compared with those in groups that were non-induced or continuously induced for the duration of the experiment. A significant difference between the pGTRTL plus doxycycline group and the non-induced pGTRTL group (P ≤ 0.05) is indicated (*), and a significant difference between the pGTRTL doxycycline removed group and the pGTRTL doxycycline 200 group (P ≤ 0.05) is indicated ($). RLU, relative light unit.